Colonel Turner was a British Army officer who along with Vice-Admiral Rolland concocted the daring plan to rescue General Carnaby. The true objective was to obtain the names of German spies working in Britain, and both Rolland and mission leader Major John Smith suspected Turner himself was Germany's number one British spy.
And indeed the Colonel was secretly a German undercover agent. Three of his handpicked soldiers for the Carnaby rescue mission, Olaf Christiansen, Lee Thomas and Ted Berkeley, were also German spies who were instructed to kill the other members of the team to prevent Carnaby's rescue. Following what seemed to be the successful completion of the mission, Turner flew with Wing Commander Carpenter to Germany to extract Smith's team.
Before he left, he was given a Sten machine gun by Rolland. After they rescued Smith's group and were returning to Britain, Smith revealed that he knew thanks to the lists Christiansen, Thomas and Berkeley had given him that Turner was a spy. Turner moved to shoot him, only to find, to his dismay, that Rolland had filed the firing pin off of the Sten he'd given him. Facing execution for treason, Turner instead chose to commit suicide. He opened the side door of the Lancaster bomber the group was flying in, and jumped out to his death.